The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 120 nanometers (nm). EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features, for example, sub-32 nm features, in substrates such as silicon wafers. To be commercially useful, it is desirable that these systems be highly reliable and provide cost effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or duster of material having the desired line-emitting element, with a laser beam at an irradiation site. The line-emitting element may be in pure form or alloy form, for example, an alloy that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a liquid.
In some prior art LPP systems, droplets in a droplet stream are irradiated by a separate laser pulse to form a plasma from each droplet. Alternatively, some prior art systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulse. In some cases, each droplet may be exposed to a so-called “pre-pulse” to heat, expand, gasify, vaporize, and/or ionize the target material and/or generate a weak plasma, followed by a so-called “main pulse” to generate a strong plasma and convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission. It will be appreciated that more than one pre-pulse may be used and more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent.
Since EUV output power in an LPP system generally scales with the drive laser power that irradiates the target material, in some cases it may also be considered desirable to employ an arrangement including a relatively low-power oscillator, or “seed laser,” and one or more amplifiers to amplify the pulses from the seed laser. The use of a large amplifier allows for the use of the seed laser while still providing the relatively high power pukes used in the LPP process.
However, even with the use of a seed laser, it is still desirable to generate a sufficiently large laser pulse so that the need for amplification can be limited. Suppose that a design calls for pulses of 1 kW (1,000 watts) from a seed laser, which are then amplified. One could generate such a 1 kW pulse by using a 1 kW laser in continuous mode, i.e. constant output, and passing the resulting beam through a very fast shutter. Such a solution would be extremely wasteful, as the typical duty cycle, i.e., the ratio between the duration of the pulse to the time between pulses, is very low, typically on the order of 1%. Thus, 99% of the output power of the laser would be wasted.
For this reason and others, various techniques of obtaining better utilization of laser output have been developed in which the laser does not operate continuously but rather builds up power until a pulse is released. One well-known technique is Q-switching, sometimes known as giant pulse formation, which allows a laser to produce pulses of much greater power than if the laser were operated in continuous mode.
Q-switching is achieved by putting some type of variable attenuator inside the laser's optical cavity (the “Q-switch”) that is externally controlled. The Q-switch functions as a type of shutter, and may for example be an acousto-optic module (AOM) that can be adjusted by the application of a control signal to pass differing amounts of the light incident upon it. The Q-switch is initially closed, which prevents the laser from lasing and allows the energy stored in the laser medium to increase. The Q-switch is then quickly opened, allowing for all of the built up energy to be released in a relatively short pulse.
For example, using Q-switching, a laser might generate pulses that are each ½ microsecond (μs) long at a rate of 50,000 to 100,000 times per second, thus allowing power to build up for 10 to 20 μs between pulses. In this way, a laser that would generate 50 watts in continuous mode may generate pulses of 500 watts to 1 kW.
However, Q-switching with an infrared laser, such as a CO2 laser, suffers from another problem. When the Q-switch is opened, allowing lasing to occur, there is a statistical uncertainty as to when the first photons will be emitted within the cavity, so that the precise timing of when the pulse will be generated is not predictable. Typically there will be nothing for 100 to 200 nanoseconds (ns), and sometimes as long as 400 ns. This “temporal fitter” is not a shutter problem, as operation of the Q-switch is predictable while the beginning of lasing is not.
A known modification of Q-switching is to have the seed laser “pre-lase,” i.e., to lase at a low level that does not use all of the power building up in the seed laser. In this case, the Q-switch is not “completely closed” as above, but rather provides partial attenuation of the laser energy. The amount of attenuation present before the Q-switch is opened determines the “lasing threshold,” the level at which a pre-pulse is created and laser oscillation builds up rather than dies; the less attenuation there is by the Q-switch (“decreasing Q-switch drive”), the lower the lasing threshold and the faster the pre-pulse starts. The Q-switch is ideally set at a level that does not use much power so that power may build up in the seed laser. The Q-switch is then fully opened, allowing all of the power that has built up in the seed laser to generate a large pulse.
Pre-lasing also suffers from temporal jitter, but as long as pre-lasing occurs, a larger pulse will occur when the shutter is opened. Thus, the timing of the large puke is much more predictable than in ordinary Q-switching. The cost of this is reduced power; if a laser can produce a pulse of 1 kW with ordinary Q-switching, it might produce only 500 watts when pre-lasing is used.
Pre-lasing suffers from a different timing problem, however. If pre-lasing occurs too early, gain of the seed laser will be reduced. If pre-lasing occurs too late, it may not occur before the Q-switch is opened, and no lasing will occur at all in the seed laser.
There is also another problem that is separate from, and not solved by, Q-switching or pre-lasing. As is known in the art, a laser has a number of possible “cavity modes” at certain frequencies that depend upon the length of the laser cavity. If the relationship of those frequencies to the laser's gain changes due to a change in the cavity length, for example due to thermal effects, the available power of the laser can decrease significantly. A change in cavity length of even a few microns can have a substantial effect on the seed laser output power.
Accordingly, it is desirable to have an improved system and method for stabilizing a seed laser by controlling both cavity length and pre-lasing while still producing periodic pulses such that the seed laser output power is maximized for use in such an EUV light source.